Structures and energies of extended defects and interstitial elements in crystals have been sensitive benchmarks in the development and validation of interatomic potentials for atomistic simulations of non-magnetic transition metals. In this lecture, our recent efforts to construct tight-binding total-energy models (TB) and bond-order potentials (BOP) from first-principles density-functional-theory (DFT) calculations for magnetic phases of iron, pure or with interstitial hydrogen or carbon, will be presented. The models are obtained from a DFT data base of properties for bulk structures and compositions. Their quality for describing atomic and extended defects will be discussed by comparing TB and BOP results with DFT targets and experimental results for properties of interstitial H and C, grain boundaries and dislocations in magnetic iron phases.